A dc rotating machine typically includes a rotor surrounded by a wound stator. A rotor connected commutator with copper segments and stationary brushgear are used to control the commutation of current in the rotor winding based on the angular position of the rotor. In a further development of the dc rotating machine, an electronic switching circuit is used to control the commutation of current in the stator winding based on the angular position of the rotor. The rotor provides a rotating magnetic field and this can be generated by permanent magnets, conventional windings with a slip ring or brushless excitation power supply, or superconducting windings with a suitable excitation power supply.
British Patent Application 2117580 discloses a brushless dc rotating machine that employs an electronic switching circuit. The electronic switching circuit essentially replicates a conventional brush and commutator topology for an industry-standard lap wound stator where the brushes are replaced by thyristors and the commutator segments are replaced by a point of common coupling between associated pairs of thyristors. One thyristor in each thyristor pair has its anode connected to a first dc terminal, while the other thyristor in that pair has its cathode connected to a second dc terminal. A stator winding with n series connected coils has n nodes that intercept the n points of common coupling in the electronic switching circuit. This is illustrated in FIG. 1 for the case where n=8.
It can seen from FIG. 1 that a first coil C1 is connected to a second coil C2 and to a first pair of thyristor switches S1a and S1b by means of a first point of common coupling PCC1. The anode of thyristor S1a is connected to a first dc terminal DC1 by means of a first ring “Ring 1” and the cathode of thyristor S1b is connected to a second dc terminal DC2 by means of a second ring “Ring 2”. The second coil C2 is connected to a third winding C3 and to a second pair of thyristor switches S2a and S2b by a second point of common coupling PCC2. The anode of thyristor S2a is connected to the first dc terminal DC1 by means of the first ring “Ring 1” and the cathode of thyristor S2b is connected to the second dc terminal DC2 by means of the second ring “Ring 2”. The remaining coils are connected to the first dc terminal DC1 and the second dc terminal DC2 in a corresponding manner.
The thyristor pairs are commutated by the back EMF (i.e. the electromotive force induced in the stator windings) of the brushless dc rotating machine and by the application of gate pulses that are synchronised with the angular position of the rotor. At low rotor speeds this back EMF can be insufficient for the commutation of the thyristors and an external commutation circuit of the dc line type is employed. This external commutation circuit is also synchronised to the angular position of the rotor and hence to the gating of the thyristor pairs.
The brushless dc rotating machine of British Patent Application 2117580 suffers from the following disadvantages:                (i) the synchronisation of the gating of the thyristor pairs to the angular position of the rotor is difficult to implement and is prone to electromagnetic interference; and        (ii) the gating synchronisation of the thyristor pairs and the subsequent commutation process is not capable of precisely adapting to the load conditions of the machine.        
It will be readily understood that it is possible to derive a general linear machine equivalent of the brushless dc rotating machine described in British Patent Application 2117580. This brushless dc rotating machine is depicted in FIG. 1 as having a pair of ring-type dc terminals and a circular array of switching stages but these can be represented in a linear form for ease of clarity.
The linear form of a basic stator winding (polygonal winding or lap winding) and electronic commutator circuit is shown in FIG. 2. The electronic commutator circuit consists of a number of identical switching stages disposed in a linear rather than a circular array. Each switching stage includes a pair of thyristors 1 and 2 as shown in FIG. 3. During operation of a dc electrical machine in a generating mode, for the majority of the time, dc current flows into the winding at a first point in the array and flows out of the winding at a second point in the array. The second point is displaced approximately 180 electrical degrees from the first point. The winding current diverges into two approximately equal paths at the first point and recombines at the second point, as shown in FIG. 2. In order to effect commutation, the location of the first point or the second point must be indexed one step along the array. The indexing may occur simultaneously, from the first point to a third point and from the second point to a fourth point, or the indexing may be sequential, with initial indexing from the first point to the third point followed by indexing from the second point to the fourth point. According to well-known natural commutation terminology, a switching device that carries current after the commutation is known as an incoming device and the device that carries current before the commutation is known as an outgoing device. The condition of these two devices is subject to overlap and reverse recovery, and phase control may be employed. In this case, the indexing is caused by the application of a gate pulse to the incoming device at a time when its terminals are forward biased (anode positive with respect to the cathode) and in so doing causing the power terminals of the outgoing device to be reverse biased.
A generic commutation process for the incoming and outgoing thyristor of adjacent switching stages is shown in FIGS. 4 and 5. FIG. 4 depicts the equivalent circuit definitions and FIG. 5 shows idealised commutation waveforms.
With reference to FIG. 4, a constant current source “I load” represents the current to be commutated out of the outgoing thyristor and into the incoming thyristor. The rate of change of commutated current is determined by the equivalent circuit representation of the stator winding and comprises a time-variable commuting voltage E and a commutating reactance having an inductance L. The outgoing thyristor carries a current “I out” and supports voltage “V out”. The incoming thyristor carries current “I in” and supports voltage “V in”.
The outgoing thyristor is considered to be latched on but un-gated at the start of the commutation process. When the incoming thyristor is gated on, the voltage “V in” collapses and current “I in” rises at a rate E/L. Since the current source “I Load” is constant, the current “I out” reduces at a rate E/L. The time taken for the current “I out” to reduce to zero is defined as the overlap. At the end of the overlap, the turn-off process is initiated in the outgoing thyristor. The turn-off process begins with the reverse recovery of the outgoing thyristor; initially the outgoing thyristor conducts in reverse, thereby causing the current “I in” to overshoot and the rate of change of current is determined by E/L. Eventually, towards the end of reverse recovery, the outgoing thyristor conductivity reduces causing the voltage “V out” to increase towards the time-variable commuting voltage −E. The effect of increasing the voltage “V out” is to force current out of the outgoing thyristor. At this time, the outgoing thyristor has not attained significant forward blocking capability, but it does have a reverse blocking capability. The voltage “V out” therefore overshoots as a result of the interruption of current in the stator winding. As commutation continues, the outgoing thyristor steady-state and transient forward voltage blocking capability increase.
Eventually, the voltage “V out” reverses and the outgoing thyristor must support this voltage. The total time available for the outgoing thyristor to attain the required forward voltage blocking capability is known as the turn-off time Tq. This is the time between the negative going reversal of the current “I out” and the subsequent positive going reversal in the voltage “V out”. If the turn-off time Tq is sufficient then the current “I out” remains at zero while the voltage “V out” tracks the time-variable commuting voltage E. However, if the turn-off time Tq is not sufficient then the current “I out” is re-established, the voltage “V out” collapses and the outgoing thyristor fails to turn off. This failure is sometimes called as “Tq failure” and can result in the destruction of the outgoing thyristor with the catastrophic malfunction in the associated equipment.
Only one thyristor of each switching stage is shown in FIG. 4 because in the interest of clarity the load current “I load” is assumed to be unidirectional (that is from left to right as drawn). Both of the thyristors have the same polarity such that thyristor current flows from right to left as drawn, except during reverse recovery. However, in cases where the load current is subject to periodic reversal then each switching stage will incorporate a pair of thyristors of opposite polarity.
The commutating voltage E in FIG. 4 is time-variable and is typically sinusoidal. The point in time at which the commutation is initiated may be adjusted in phase, with respect to the waveform of the commutating voltage E, according to the well known technique of phase control. In principle, naturally commutated thyristors are ideal where the brushless dc rotating machine is used for motoring applications. However, performance limitations are significant when operating at low speeds because the optimum points of current entry into the stator winding are sited where there is inadequate coil voltage to cause reliable commutation. At high motoring torque and low speed the resulting commutation overlap duration leaves inadequate time for the thyristors to attain a forward blocking capability. External dc line commutation apparatus can be employed to overcome the limitations of natural commutation, but this can cause undesirable high amplitude torque pulsations and additional power losses to be generated.